Activation of the complement system (C) is accomplished via three different initiation pathways: The alternative (AP), the classical (CP), or the lectin pathway (LCP).
AP activation occurs on foreign surfaces and is caused by a slow, spontaneous hydrolysis of C3 and the activity of the factors properdin, factor B and factor D to form the functional C3 convertase C3bBb. AP also functions as an amplification pathway (the amplification loop) of the two other pathways. Recently it has been shown that the alternative convertase assembly may also be initiated by non-covalent attachment of properdin to some target surfaces. CP activation on the other hand is initiated when C1q binds to immunoglobulins in complex with antigens, which triggers the activation of the C1q-associated serine proteases C1r and C1s. C1s cleaves and activates C4 and C2 to form the CP C3 convertase C4b2a. The LCP is activated when mannose-binding lectin (MBL) or ficolins binds to restricted patterns of carbohydrates or acetylated compounds e.g. on the surface of microorganisms or when exposed on dying host cells. Upon binding to the ligand the associated serine protease MASP-2 activates and cleaves C4 and C2 to form the LCP C3 convertase C4b2a. The function of MASP-1 has been suggested to involve a stabilization of MASP-2 cleavage of C2 and also direct low grade cleavage of C3. Yet other studies relate the function and activity of MASP-1 and MASP-2 to a coagulation system cross-talk involving prothrombin, fibrinogen and factor XIII. Using MASP1/3 knockout mice it was recently demonstrated that MASP-1 in fact contributes to the complement activity. The exact function of the most recently discovered MBL associated serine protease MASP-3 has yet to be elucidated. Studies indicating that MASP-3 associates with a limited range of MBL oligomers and that MASP-3 and the small MBL-associated protein (sMAP) are involved in regulation or inhibition of MBL dependent LCP complement activation have been reported.
MASP-1 and -3 are derived from the same MASP1/3 gene (present on chromosome 3q27-q28) through differential splicing. They contain an identical A-chain except for 15 C-terminal residues. The A chain is comprised of two CUB (C1r/C1s, Urchin-EGF, Bone morphogenetic protein) domains separated by an EGF (Epidermal Growth Factor) domain and followed by two CCP domains (complement control protein). The B-chain including the serine protease domain is different for MASP-1 and MASP-3. The MASP-2 and sMAP are also derived from the same gene (present on chromosome 1p36-p36.2) where sMAP is a truncated form lacking the serine protease domain and a major part of the A-chain. The MASP1/3 gene has been shown to be polymorphic, but the functional importance of this is still poorly understood. However, there is some evidence that polymorphisms in the MASP2/sMAP gene are associated with increased risk of infections. Expression of the MASPs is localized to liver hepatocytes, but a recent study described that human MASP-3 mRNA (as the only MASP-mRNA) was expressed in a broad range of tissues.